Polyhalogeno-allenes and -acetylenes. Part XII[1]. Further studies on routes to tetrafluoroallene and tetrafluoropropyne

JournalofFIuorineChemis~, 10 (1977)487-493 0 Ekwier Sequoia S.A., Lausanne - Printed in the Netherlands

487

Received: July 5,1977

POLYHALOGENO-ALLENES

AND -ACETYLENES. PART XII[l]. FURTHER STUDIES ON ROUTES TO TETRAFLJJOROALLENE AND TETRAFLUOROPROPYNE

R.E. BANKS, W.D. DAVIES, R.N. HASZELDINE and D.R. TAYLOR Chemistry Department, The University of Manchester Institute of Science and Technology, Manchester M60 l&D (Great Britain)

SUMMARY Passage of the 2,3-dibromopropene CF2BrCBr=CF2 over hot catalytic grade silica causes it to isomerize to its 1,2-dibromo analogue CFTCBr=CFBr.

Flow pyrolysis of these dibromopropenes over copper yields tetrafluoroallene (39%) and tetrafluoropropyne

(11s).

Isomerization (with catalytic

grade silica, boron tribromide, or aluminium tri-bromide or -chloride) of the 3-bromopropene CF2BrCH=CF2 followed by KOHd&ydrohalogenatiOnof the resulting (s)/(z) mixture of the 1-bromopropene CF3CH=CFBr gives low yields of the propynes CF3CECBr

and CF3CECF.

INTRODUCTION Difficulties associated with the synthesis of tetrafluoroallene and tetrafluoropropyne have seriously hindered studies on these compounds.

Tetrafluoroallene is best

prepared by the five-stage route CF2Br2 + CH2=CF2+ CF2BrCH=CF2 -CF2BrCHBrCF2Br~

CF2ErCH2CF2Br-

CF2BrCBr=CF2 (I) -

CF2=C=CF2 [2], the first four of

which are common to the synthesis of tetrafluoropropyne CF3CBr=CFPr (II) (I) c ---+CF2BrCBr=CF2 CF3C=WF

[2] l. Eff orts to improve these methods led to

the results now presented.

488 RESULTS AND DISCUSSION The final stages in the preferred (see above) syntheses of tetrafluoroallene and tetrafluoropropyne are batchwise debrominations, effected with magnesium in tetrahydrof'uran (74% yield of CF2=C=CF2) and zinc in dioxan (43% yield of CF3CsCF),

respectively [2l.

Attempts were made to improve

both processes in terms of yield and throughput by using hot copper in a flow apparatus as the bromine acceptor:

a

similar technique was employed previously [31 to effect the debromination CF2BrCFBrSF5+ (conversion 56%).

CF2=CFSF5 in 88% yield

Despite numerous experiments, however, the

best yield of tetrafluoroallene was only 39%, and acceptable conversions (360%) not be achieved.

of 2,3-dibromotetrafluoropropene

(I) could

Debromination of 1,2-dibromotetrafluoro-

propene (II) by this method also proved unsatisfactory:

good

conversions (ca. 70%) were achieved but yields of tetrafluoropropyne were poor (ca. 10%). During the search for ways to increase the efficiency of the copper-initiated debromination of 2,3-dibromotetrafluoropropene, it was observed that isomerization of the dibromocompound to 1,2-dibromo-1,3,3,3-tetrafluoropene occurred (CF2Br~~rl~~2 .ab CF,$Er&FBr)when mixtures of copper and catalytic grade silica were used.

Quantitative conversion of

the 2,3-dibromide to a ca. 3:2 mixture (65% yield) of (z)and (E)-1,2-dibromotetrafluoropene was achieved by passing it at 5 mmHg pressure over only fresh silica at 240°C, and the yield increased to 86% after the silica had become 'seasoned'. Thus an efficient flow method was developed for obtaining the immediate precursor of tetrafluoropropyne, the activity of the catalytic grade silica possibly

stemming from the traces

of alumina it contained. Previously the isomerization of 2,3to 1,2-dibromotetrafluoropropene was effected batchwise with aluminium tribromide [2-j. B=2C2 CF2Br2 + CH2=CF2 a~

CF2BrCH=CF2

4

CF2BrCH2CF2Fr

CF3CH=CFBr

C a =

-HBr >CF3CECF

489

In an attempt to shorten the six-step synthesis [2] of tetrafluoropropyne, the possibility of branching the route to tetrafluoroallene at the 3-bromo-1,1,3,3-tetrafluoropropene stage as shown above was examined.

Efficient

isomerization of the 3-bromo-compound to a ca. 3:2 mixture of (E_)-and (Z)-l-bromo-1,3,3,3-tetrafluoropropene (isomers which, under certain conditions are formed in small amounts during the carbon-catalysed thermal dehydrobromination 1,3-dibromo-1,1,3,3-tetrafluoropropane

of

in step 2) was effected

with boron tribromide (92% yield), aluminium halides (AlEr , 91%;

A1C13, 91%),

or

fresh catalytic grade silica

(71%): Dehydrohalogenation of the mixture of (E_)-and (Z)-l-bromo-1,3,3,3-tetrafluoropropene with solid potassium hydroxide at 120°C, however, gave tetrafluoropropyne in only 1% yield, the major product being l-bromo-3,3,3trifluoropropyne

(15%).

Separate experiments with the two

geometric isomers of the bromopropene indicated that the major source of the bromopropyne was the fE)-isomer, i.e. that a trans-elimination mechanism had predominated.

EXPERIMENTAL '9F N.m.r. spectra were obtained with a Perkin-Elmer R10 (56.46 MHz) spectrometer using external CF3 C02H as reference (low field shifts designated positive).

Unless

stated otherwise, products were identified only by g.1.c. and i.r. spectroscopy.

Debrominations (a) 2,3-Dibromo-1,1,3,3-tetrafluoropropene The propene (1.05 g, 3.86 mmol) was passed at 5 mmRg pressure during 1.5 h through a silica tube (80 cm x 1 cm. i.d.), the central section (38 cm) of which was packed with copper turnings and heated to 26O'C. The product, collected was fractionated, in vacua, to give tetrafluoroat -196OC,

490

allene (0.024 g, 0.217

mmol, 39%) and starting material

(0.900 g, 3.30 mmol, 86% recovery). Passage of 2,3-dibromotetrafluoropropene

(1.46

g,

5.36 mmol) at 5 mmHg pressure during 3 h through a hot (37O'C) copper tube (0.8 cm i.d.) packed with copper turnings (heated length 50 cm) gave tetrafluoroallene

(0.124

mmol, 33%) and starting material (4.99 mmol, 93%). (b) 1,2-Dibromo-1,3,3,3-tetrafluoropropene The propene (1.08 g, 3.97 mmol) was passed at 15 mmHg pressure during 5 min through a silica tube (80 x 1 cm i.d.), the central section (15 cm) of which was packed with copper bronze supported on copper turnings and heated to 33O'C. Fractionation of the oroduct gave tetrafluoropropyne (0.036 g, 0.31 mmol,ll% based on

CF3CBr=CFBr consumed)

contaminated (ca. 2% by i.r. analysis) with tetrafluoroallene, and starting material (0.345 g, 32% recovery).

Isomerizations (a) 2,3-Dibromo-7,1,3,3-tetrafluoropropene The propene (2.09 g) was passed at 5 mmHg pressure during 2 h through a pyrolysis train comprising a Pyrex preheater (50 cm x 0.8 cm i.d., 100°C) attached to a silica tube (80 cm x 1 cm i.d.) the central section (50 cm) of which was packed with a 1~4 w/w mixture of copper turnings and catalytic-grade silica (particle size 5 mm: Unilever Ltd., wide-pore gel, 380 m2/g) and heated to 245'C. The product, collected at -196'C, was shown by lgF n.m.r. spectroscopy to be a 59:41 mixture of (z)- and (E)-1,2dibromo-1,3,3,3-tetrafluoropropene

(1.39 g, 67%):

Pyrolysis of 2,3-dibromo-1,1,3,3-tetrafluoropropene

(2910 g)

at 240°C during 45 min as above, but with the silica tube packed with only fresh silica particles, also gave only a ca. 3:2 mixture of (g)- and (E)-1,2-dibromotetrafluoro_ Lopenes

(1.37 g, 65%):

yellowish brown.

the silica packing turned

Pyrolysis of 2,3-dibromotetrafluoro-

491

propene (7.20 g) during 7 h at 235'C and 5 mmHg pressure over 'seasoned' silica (used four times previously) gave 1,2-dibromotetrafluoropropenes

(6.20 g) in 86% yield.

(b) 3-5romo-1,1,3,3-tetrafluoropropene (i) With silica.

Passage of the propene (1.47 g)

over fresh catalytic-grade silica [as in (a) above] at 240°C and 5 mmHg pressure during 45 min gave a mixture (16:9 by "F

n.m.r. spectroscopy) of (E_)-and (Z)-?_bromo-

1,3,3,3-tetrafluoropropene

Cl.05 g, 71%).

(ii) With boron tribromide. 1,1,3,3-tetrafluoropropene

A mixture of 3-bromo-

(4.911 g, 25.40 mmol) and boron

tribromide (0.450 g, 1.79 mmol) was stored at 20°C in a Pyrex ampoule (10 cm3) in the absence of air for 48 h. The product was a ca. 3:2 mixture of (E_)-and (2)-lbromo-1,3,3,3-tetrafluoropropene

(4.517

(iii) With aluminium trihalides.

g,

92%).

3-Eromo-l,1,3,3-

tetrafluoropropene

(8.14 g, 42.2 mmol) was condensed, in vacua, on to anhydrous aluminium tribromide (1.9 g, 7.1 mmol) contained in a Pyrex tube (60 cm3) at -196'C. The tube was sealed, allowed to warm to 20°C, and then shaken for 24 h.

The volatile product was a ca. 3:2 mixture of (E_)-and (Z)-l-bromo-1,3,3,3-tetrafluoropropene (7.39 g, 91%). Similarly, 3-bromo-1 ,I ,3,3-tetrafluoropropene (19.68 g, 0.102 mol) and aluminium chloride (1.5 g, 11 mmol) interacted exothermically at room temperature to yield a 15:9 mixture of (E)- and (Z)-l-bromo-1,3,3,3-tetrafluoropropene gG%).

(17.82 g,

The following ‘9 F n.m.r. parameters were obtained by

examination of a 16:9

mixture of (g)- and 'Z)-l-bromo-

1,3,3,3-tetrafluoropropene: (CFFr;

(I) (E)-isomer:

6,22.3

]LF,HI

26.9, )zF,CF 1 17.2 Hz) and 17.7 p.p.m. 6.7 Hz), and324.55: (ii) !Z)-isomer: ItiF HI 21.03iCFRr: ILF HI 10.8, ILF,CF 1 13.4 Hz) and

17.85 p.p.m. (CF3:

ILF3,HI

6.4 Hz), 'and

74.06.

492

Dehydrobromination

of 1,3-Dlbromo-1,1,3,3-tetrafluoropropane

The propane (6.90 g, 25.2 mmol) was pyrolysed at 290°C and 1 mmHg pressure during 1 h as described previously [2] except that the granular charcoal was contained in a new Pyrex tube.

Separation of the product by g.1.c. f4 m

dimethylsulpholane-Celite, tetrafluoropropene

1,3,3,3-tetrafluoropropene [Found:

c,

4O'C) afforded 3-bromo-1,1,3,3-

(2.79 g, 14.4 mmol, 57$), (Z)-l-bromo-

18.8;

H, 0.8%:

requires C, 18.7;

H, 0.5%:

(nc) (0.155 g, C.80-mmol, 3%) y (Regnault) 193.

C3HBrF4 M, 1931, b.p. 31.6'C

(isoteniscope), Amax (vapour) 5.93 Wrn (C=C str.), and m/e 194 (C H8'BrF +* '62) 192 (C H7gBrF +*, 65%) , 4 69 (CF;', ?OO%.), 4 and ' (E_)-l-bromz-1,3,3,3_tetrafluoro' ' propene (nc) (0.155 g, 0.80 mmol, 3s) H, 0.8: 5 (Regnault) 193. H, 0.5%:

[Found:

c, 18.8;

C3HBrF4 reauires C, 18.7;

rJ, 1931, b.p. 36.00~ (isoteniscope), h,,,.

(vapour)

5.95 Wrn (C=C str.).

Dehydrohalogenation

of l-Bromo-1,3,3,3-tetrafluoropropene

A 3:2 mixture of (E)- and (Z)-I-bromo-1,3,3,3tetrafluoropropene (4.01 g, 20.8 mmol) contaminated with 3-bromo-1,1,3,3-tetrafluoropropene

(0.54 g, 2.8 mmol) was

added dropwise to a stirred bed of potassium hydroxide pellets (80 g) heated to q20°C in a vessel swept with a slow stream of nitrogen and attached to two cold traps (-72 and -14O'C) via a cooled (-23'C) reflux condenser. Trap-to-trap fractional condensation of the product, in vacua, afforded a mixture (-196'C trap) of 2H_pentafluoropropene (0.107 g, 0.81 mmol) (presumably derived from tetrafluoroallene), tetrafluoropropyne

(0.015 g, 0.14 mmol,

1% based on CF CH=CFBr consumed), and a trace of tetra3 fluoroallene(from the CF2ErCH=CF2 impurity), and a mixture (-95'C trap) of I-bromo-1,3,3,3-tetrafluoropropene (0.53 g, 2.75 mmol, 13% recovery) and 1-bromo-3,3,3-trifluoropropyne (0.475 g, 2.75 mmol, 15%). A sample of l-bromo3,3,3-trifluoropropyne, b.p. (isoteniscope) 26.8'~ (lit. [4],

493

25-25.5'C), Ama, (peaks with

(vapour) 4.47 pm (CEC

>30$‘rel.

abund.) 174 (C3*'BrF3+*, 87), 172 33),

‘53

loo), 55

(C3F+,

str.), 42

93

(cj7’BrF2+, !c3F3’,

35),

*7),6g

Cc~~c'52),

71), 43 (C,F+, 54@), was isolated by g.1.c.

(4 m dimethylsulpholane-Celite, 4O'C). When a mixture of powdered potassium hydroxide (78 and (Z)-I-bromo-1,3,3,3-tetrafluoropropene

g)

(1.08 g, 5.62 mmol)

was heated at gO°C for 30 min., the product comprised 2H-pentafluoropropene

(0.08 mol, 2% based on CF3CH:CFBr

consumed), tetrafluoropropyne

(0.12 mmol, 3$), a trace of

1-bromo-3,3,3-trifluoropropyne, (1.38 mmol, 255 recovery),

and starting material

Similar treatment of (E)-1(14.4 mmol) provided

bromo-1,3,3,3-tetrafluoropropene l-bromo-3,3,3-trifluoropropyne

(0.28

mmol, 2% based on

CF3CH=CFBr consumed), a trace of tetrafluoropropyne, and starting material (2.22 mmol, 15%).

REFERENCES 1

Part XI, P.W.L. Fosbury, R. Fields, and R.N. Haszeldine, paper submitted *o ;3:Chem.Soc.

2

R.E. Banks, M.G. Barlow, W.D. Davies, R.N. Haszeldine

3

R.E. Banks, M.G. Barlow, R.N. Haszeldine and

and D.R. Taylor, J. Chem. SOC. (C), (1969) 1104. W.D. Morton, J.C.S. Perkin Trans. I, (1974) 1266. 4

A.M. Shchekotikhin, V.S. Blagoveshchenskii, V.V. Sidorenko and O.K. Denisov, Zhur. Vses. Khim. Obshchestva im. D.I. Mendeleeva, 7 (1962), 580 CChem. Abs., s,

(1963) 568Oa-j.